1. Field of the Present Invention
The present invention relates generally to the field of imaging, and more particularly to the field of non-linear coherent imaging.
2. History of the Related Art
Non-linear coherent imaging refers to any imaging technique in which multiple input photons are coherently combined to produce each output photon. In general, in any optically non-linear medium, photons of all sum and difference frequencies will be produced. For example, FIG. 1 illustrates an energy level diagram for a process known as Coherent Anti-Stokes Raman Spectroscopy (CARS) in which a vibrational state of a target molecule results in the resonant enhancement of photon generation at a frequency 2ω1−ω2. In general the strength of any signal produced at a given sum/difference frequency depends on whether molecular or atomic energy levels can give rise to resonant enhancement. The strengths of different sum/difference signals depend on the detailed vibrational and electronic energy levels of the molecule being probed. For example, a process called Stimulated Parametric Fluorescence shown in FIG. 2 will be stronger when the molecule or material has an energy level close to the top level shown therein. In such a case, the existence of a molecular excited state with energy near the top level in FIG. 2 will resonantly enhance an output a 2ω2−ω1. FIGS. 3A and 3B illustrate second and third harmonic signals generated by a single input frequency, ω1.
In spite of the numerous enhancements in spectroscopy and microscopy in the recent past, the aforementioned processes lack in their ability to generate image contrast for a sample illuminated with coherent light. Given the inherent complexity in non-linear coherent imaging, it is desirable to provide image contrast in a manner that is both cost-effective and easily reproducible. As such, there is a need in the art for a coherent imaging system and/or method that is capable of generating image contrast in an efficient and effective manner.